A complete printing process, starting for instance from an image displayed on a screen (source image), requires that a computer or a micro-processor duly programmed performs a plurality of conversion steps in order to obtain, starting from an additive primary colours image or RGB (Red, Green, Blue) image, a corresponding subtractive primary colours image or CMYK (Cyan, Magenta, Yellow, Key black) image suitable to be printed by using ink dots.
The conversion steps, as for instance disclosed in Patent Publication WO2009/031165 in the name of the Applicant, comprise, for example:                a calibration process wherein a weight is assigned to each point of the source image and a corresponding set of subtractive colours image is generated;        a halftoning process wherein subtractive colours are arranged in order to obtain on the printed media an optical effect similar to that visible on the screen;        a printing process wherein, according to the result of the halftoning process and a certain printing strategy, CMY and, optionally, K dots are ejected by a print head on a printable media.        
The printing process and in particular the printing strategy (shingling strategy), in general, requires that the computer duly programmed avoids coalescence, in particular in cases where plastic cards need to be printed by using ink jet printing, because drying time of dots is very long.
As known, when ink dot are ejected on a media, a problem of coalescence may exist if not dried dots (dots) superimpose on each other.
Coalescence problem, in general, may be due to two different superimposing situations or problems:                adjacent dots superimpose during print head dots ejection; in other words the problem can happen if the print head ejects superimposing dots during a carriage pass (a carriage pass is defined as a single travel of the print head from one edge of the media to the other);        dots ejected in a pass following a previous pass superimpose previous dots before drying thereof.        
According to known prior art the first coalescence problem is solved through a shingling strategy based on a raster grid, where each pixel (and therefore the dot deposited on that pixel) is assigned to various layers so as to avoid coalescence; according to present disclosure the term layer is assumed to represent an image that is printed with several non overlapping print swaths or passes. For instance, FIG. 1 shows a two layer shingling strategy or procedure where the dots on a grey square or pixel are printed in a first layer and those on a white square are printed in a second layer so that coalescence is avoided.
In practical cases the above strategy requires, for instance, that:                assuming that dots of each layer must have a certain minimum distance, for instance Dmin=84 μm;        assuming that printer is using an asymmetric printing resolution of 600vert×1200horiz dots per inch (dpi) whereby a rectangular pixel is defined having dimensions of 42vert×21horiz μm as shown in FIG. 2a; an eight layer shingling strategy will be optimal to obtain full coverage because all dots will be printed at exactly 84 μm from each other.        
In other words the eight layer shingling strategy is optimal for obtaining a full coverage but may not be optimal in cases where:                full coverage is not a necessity in order to reach colour saturation; or        print time is an issue and the number of layers needs to be reduced by using a coverage lower than full coverage.        
For instance, assuming that 6/8 of the full coverage could be enough, one could expect 6 layers to be sufficient in order to avoid coalescence. But it is apparent that by using the prior art strategy with 6 layers instead of 8, the required distance between dots (FIG. 2b) will not be enforced and the first coalescence problem will appear. To be more precise, we consider FIGS. 2a and 2b which both exemplify prior art shingling strategy when printing at a resolution of 600vert×1200horiz dots per inch, using 8 and 6 shingling passes, respectively. In both cases, the pixel raster grid is represented and pixels are numbered according to the pass to which they belong to. Meaning that dots falling on pixels numbered with are printed on the first shingling pass, pixels numbered with 2 are printed on the second pass, and so on. Considering that, at a resolution of 600vert×1200horiz dots per inch, the pixel size is 42vert×21horiz μm, and considering also that 6/8 of all pixels will be occupied by a dot in a pseudo random way, it becomes evident that:                on FIG. 2a, the prior art 8 passes shingling strategy always enforce the required coalescence distance of 84 μm.        on FIG. 2b, the prior art 6 passes shingling strategy only does enforce a distance of 63 μm which does not prevent coalescence.        
In summary, Applicant has noted, in general, that known prior art does not optimally solve the first coalescence problem. “Optimally” means that, in order to avoid the first coalescence problem, the classic shingling strategy needs a number of layers larger than due.
Applicant has noted, moreover, that the first coalescence problem is an issue in cases where plastic cards need to be printed.